Description
This issue aims to fix a build error on branch 1.6, but we had better have this in master branch, too. There are other ongoing PR, too.
[error] /home/jenkins/workspace/spark-branch-1.6-compile-maven-with-yarn-2.3/sql/core/src/test/scala/org/apache/spark/sql/StringFunctionsSuite.scala:82: value toDF is not a member of Seq[String] [error] val df = Seq("aaaac").toDF("s") [error] ^